bump version to 12.2.2-pve1

[ceph.git] / ceph / src / boost / libs / vmd / doc / vmd_dynamic_typing.qbk

[/ 
  (C) Copyright Edward Diener 2011-2015
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:vmd_dynamic_typing Input as dynamic types]

Within the constraints based on the top-level types which VMD can parse,
the libraries gives the end-user the ability to design macros with
dynamic data types. By this I mean that a macro could be designed to handle
different data types based on some documented agreement of different combinations
of macro input meaning slightly different things. Add to this the ability
to design such macros with variadic parameters and we have a preprocessor
system of macro creation which to a lesser extent rivals the DSELS of template 
metaprogramming. Of course the preprocessor is not nearly as flexible as
C++ templates, but still the sort of preprocessor metaprogramming one could
do with VMD, and the underlying Boost PP, in creating flexible macros which
can handle different combinations of data types is very interesting.

Of course macros need to be usable by an end-user so the syntactical ability
of sequences to represent different types of input data must
be balanced against ease of use and understanding when using a macro. But because
certain sequences can mimic C++ function calls to some extent it is possible
to represent macros as a language closer to C++ with VMD.

What is important when designing a macro in which you parse input to decide which type
of data the invoker is passing to your macro is that you are aware of the constraints
when parsing a data type. As an example if you design a macro where some input can either
be a number, an identifier, or some other data type top-level input then attempting 
to parse the data to see if it is a number or identifier could fail with a preprocessor
error and nullify your design if the data is not a VMD data type. So designing a macro with 
data types in mind often means restricting data to parseable top-level types.

[endsect]